Field of the Invention and Related Art Statement
The present invention relates to an apparatus for reading information out of an optical record medium such as an optical card, compact disc, video disc or data disc.
There have been proposed various apparatuses for reading the information out of the optical record medium. For instance, in Japanese Patent Publication Kokai No. 56-83,849, there is disclosed an apparatus for reading information out of an optical record disc comprising a light source for emitting a light beam, a collimator lens for converting the light beam emitted from the light source into a parallel light beam, a beam splitter for splitting the parallel light beam, an objective lens for projecting a light beam emanating from the beam splitter upon an optical record disc and collecting and guiding a light flux reflected by the optical record disc into the beam splitter, a converging lens for converging a light beam emanating from the beam splitter and a photodetector for receiving the converge light beam to generate an electric signal representing the information stored in the optical record disc.
In the above mentioned known optical reading apparatus, the light source is arranged at a focal point of the collimator lens so that the light beam emitted by the light source is converted into the parallel light beam, and the parallel light beam is focused by the objective lens onto the optical record disc. In this known optical reading apparatus, the focused spot of the light beam is made incident upon the optical record disc, it is possible to read the information track having a very small width. By processing the electric signal supplied from the photodetector, the information signal as well as tracking and focusing error signals is derived by means of which the reading light beam can follow the data track and is focused on the record disc. In order to derive the focusing error, there is provided a special optical element such as a cylindrical lens for introducing astigmatism and a prism having a reflection surface set at a critical angle with respect to an incident light flux. In the reading apparatus for the optical card, it is desirable to make the apparatus as compact as possible, requiring deletion of the special optical element. In such a case, the focusing error has to be detected by any other a different means. In Japanese Patent Publication Kokai Sho 61-153,834, described an apparatus for reading information out of an optical card. In this known apparatus, it is necessary to read simultaneously three tracks, i.e. data track, control or clock track and guide track. To this end, three beams are used each of which is made incident upon respective tracks by means of a common objective lens. Further, in order to derive the focusing error, the light beams are made incident upon the optical card from an inclined direction. However, this known apparatus has a drawback that the optical system becomes very complicated and critical due to the fact that the three light beams have to be made incident on the optical card.
Japanese Patent Publication Kokai Sho 62-279,523 described another known apparatus for reading information from the optical card. In this known apparatus, the optical card has a track construction such that a single data track comprises twenty four data lines and one control line, the twenty five lines have to be read out simultaneously. Therefore, the use of the technique disclosed in the Japanese Patent Publication Kokai Sho 61-153,834 requires twenty five light beams. However, it is practically impossible to generates such a large number of light beams. In the Japanese Patent Publication Kokai Sho 62-279,523, a light beam emitted from a light source, e.g. laser diode, is projected upon the optical card by means of an illumination lens as a light spot having a diameter sufficiently larger than a width of the data unit, and a light flux reflected by the optical card is collected by an objective lens. The illumination lens and objective lens are arranged such that their optical axes form a V-shaped configuration viewed from a direction perpendicular to an incident plane. The focusing error can be detected without providing the special optical element. However, this known apparatus has a drawback that the illuminating lens makes the cost and size of the apparatus higher and larger, respectively.
In the above mentioned known optical information reading apparatus disclosed in the Japanese Patent Publication Kokai Sho 62-279,523, the control line is formed by an array of square blocks arranged equidistantly. By detecting this array of blocks, there can be detected a control signal by means of which there is generated the clock signal as well as the focusing and tracking error signals. Therefore, in the case of one or more blocks formed incorrectly or having defects, it is not therefore possible to derive the control signal correctly, therefore producing large focusing and tracking deviations, and the information can not be read out correctly.